As Nick says - easier if you can push down. Move the bar 180 degrees! Get the subframe sat stable and wedged against something so that you can push (or stand) down heavily on a good quality long bar with the correct socket, so that there isn't any play. If it's as tight as mine was, you may even just spin the wheel if you don't have a full cars weight on it.

You need the assembly well braced, so all the force you apply is transferred to the nut. The other thing you could try is getting the nut red hot with a blow lamp which might help break it's grip. Good luck. Not that it helps you, but it is far easier with it all assembled on the car and road wheel on the ground accessing the nut through the centre of the wheel.

with the subframe like that you might be onto a looser as you really need to keep the wheel from turning with the braks on, nut is done to 150ft Lb or more, if you need to get it off it might be easiest to slit the nut with a chisel and replace it, mind the cv threads though!

I would steer clear of heating it as the heat is going to tracel up the CV and maybe cook the bearing./ seals